Internet-based activities are now subject to electronic vandalism. For example, a vandal or hacker may attack an Internet web server by flooding it with a torrential flow of disruptive messages that overload the server to the point of functional failure. Attacks of this kind are called “denial of service” attacks.
During a denial of service attack, the vandal may fraudulently assume a number of different electronic identities, often by including messages in the disruptive flow that have a variety of source addresses. To combat such attacks, a server may rely upon protective equipment that filters incoming messages. Such equipment detects the onslaught of a vandal's attack, reads the source addresses that the attacker usurps and fraudulently re-uses, and blocks all messages that purport to originate from these source addresses.
Unfortunately, the use of protective filtering may play into the hands of a vandal who resorts to “spoofing.” A spoofer is an attacker who uses a source address that fraudulently identifies the spoofer as a source that is already known to the server. Spoofing attacks may have serious consequences, for example when the spoofer usurps the source address of a web server's most important customer. In such instances, the web server's protective equipment filters-out all messages that bear the customer's source address, including messages actually sent by the customer. Consequently, the web server experiences both the trauma of an attack and the adverse consequences that come with mounting a defense that filters-out legitimate messages sent by the server's most important customer.
Thus there is a need for a defense against vandals who spoof and who launch denial-of-service attacks against a server, where the defense does not restrict legitimate use of the server.